Macroscopic Behavior of Kraft Lignin Fractions: Melt Stability Considerations for Lignin–Polyethylene Blends

Questions that pertain to the behavior of softwood kraft lignin fractions as opposed to the whole lignin still prevail. In an effort to further understand such effects at a macroscopic level, we have examined the issue of melt stability of polyethylene (PE) in blends with fractionated and unfractionated softwood kraft lignin. While methylation of the phenolic OH groups significantly stabilizes any lignin/PE melt, more in depth data demonstrate that when separate, acetone soluble (ASKL) and acetone insoluble (AIKL) kraft lignin fractions behave completely differently than the original unfractionated material. The low molecular weight methylated ASKL offers unexpected plasticizing action to PE melts, possibly as a consequence of its low molecular weight and spherical configuration. The higher molecular weight AIKL offers relatively stable PE melts, possibly manifesting its rigid nature and higher glass transition temperature likely occurring due to π stacking operating among its aromatic rings. Mixing these two fractions together creates unstable PE melts with a propensity to extensively thermally cross-link in the absence of methylation. These findings offer for the first time insight into the nonproductive cooperative action of individual fractions in softwood kraft lignin with significant technological ramifications.